Cell relay technology is emerging as the method of choice for future local and wide area communication networks. Such networks carry a wide variety of traffic types from different applications, with data, voice, image and video being the often cited examples. Using a cell relay mechanism to switch different traffic types in a network provides a means for achieving integration of both transmission and switching resources.
FIG. 1, numeral 100, illustrates the elements of a typical cell relay network. Besides the sources of traffic (voice, data, image and video) (102, 104, 106, 108, 110, 112, 114, 116, 118, 120, 122) at its edge, the network consists of nodes (128, 130, 132) for switching traffic, a network manager (124), a Broadband Integrated Services Data Network (B-ISDN, 126) (where selected), and internodal links (INLs) ( 134, 136, 138, 140) for transporting traffic between nodes (128, 130, 132). In cell relay networks, information is transported in a packet format referred to as a cell. Cell relay networks are connection-oriented, and during connection establishment, a path through the network is determined by a routing function. Following connection establishment, all cells for the connection are relayed along this path. Each node along the selected path serves to relay each arriving cell to the proper output port on the node. The output port on the node may be supporting an internodal link or, at the source or destination edge, supporting an access link to an end system (or to a server in the node that is converting between the native end system format and cells). Cells from several different connections may be multiplexed and carried on the same link.
The relaying of cells in a node makes use of a connection identifier within the cell header. FIG. 2, numeral 200, shows the format of a typical Asynchronous Transfer Mode (ATM) cell (201). The ATM cell header structure at a network node interface includes a first field that is a Virtual Path Identifier field (202), a second field that is subdivided into a VPI field (204) and a Virtual Channel Identifier field (206), a third field that is a VCI field (208), a fourth field that is subdivided into a VCI field (210), a Payload Type (PT) field (212) and a Cell Loss Priority (CLP) field (214), a fifth field that is a Header Error Control (HEC) field (216). The VPI and VCI together form the connection identifier for an ATM cell relay connection. The connection identifier is used to distinguish among cells from different connections multiplexed on the same link: all cells on a link originating from the same connection have the same connection identifier. The identifier has local significance to an ATM internodal or access link. That is, different connection identifiers may be used to identify the same connection on different links along its path. As a cell is relayed by a node from one link to another, the connection identifier is translated from the value used to identify the connection on the inbound link to that which is used to identify the connection on the outbound link. For example, a VPI/VCI of 5/7 used to identify an ATM connection on an inbound link may be translated to values 3/6 to identify the same connection on the outbound link. The cell payload (218) occupies the remainder of the ATM cell (i.e., the information field).
FIG. 3, numeral 300, is a block diagram of the typical basic structure of a cell relay node. It consists of a plurality of Cell Processors (CPs) (304, 306, . . . , 308) interconnected through a Cell Interconnect (302). Each CP supports one or more nodal interfaces, and each nodal interface may support either an access link or an internodal link. One may think of a CP as divided into two halves, one for inbound cell processing and the other for outbound cell processing. The translation of the connection identifier is typically performed by the inbound cell processing. Here the connection identifier of each cell received over an access or internodal link is used to look up a table entry that specifies the outbound link (or switch port supporting that link), along with the new (i.e., translated) connection identifier value associated with the connection on the outbound link. The inbound cell processing may also include traffic monitoring functions (to determine if a connection is violating traffic profiles agreed to during connection establishment) and, depending on the architecture of the Cell Interconnect (302), may include functions for cell queueing for cases when access to the Cell Interconnect (302) is delayed for an inbound cell.
Prior art teaches the architectural design of many different Cell Interconnects. These include an arbitrated bus, a time-division bus, a fully interconnected fabric, and a multistage interconnect. Prior art also teaches that cell queueing is a basic function of any cell switch in that one must be able to handle the case when cells arriving simultaneously on two or more inputs may be destined for the same output on the switch. Prior art also teaches that placing the queueing function at the outputs of the cell switching node avoids problems of head-of-line blocking and provides the best throughput/delay performance achievable. However, placing the queueing function at the outputs of the node (i.e., as part of the outbound cell processing function on the CPs), places a difficult processing load on the outbound side of the CP. Specifically, the instantaneous rate of cell arrivals to a CP, via the Cell Interconnect, for outbound processing may be as high as the aggregate rate of all inbound traffic to the node. Moreover, recent art further teaches the need to enqueue cells into separate queues at the output for connections of different traffic types (constant bit rate, voice and data) that are multiplexed onto a link. The order in which cells are selected for transmission on the link from the individual queues constitutes the service discipline for the queueing system. In addition, the service discipline may need to discard cells (e.g., ATM cells with the Cell Loss Priority (CLP) bit set) during times of congestion (i.e., when one or more of the queues grows significantly). Performing all the required cell processing in the outbound direction at the potentially high rate at which cells may arrive to the CP from the Cell Interconnect is difficult and costly to achieve.
Thus, there is a need for a cost-efficient cell processing device and method that facilitates cell transmission from a cell interconnect to a node output.